Realizing that something was wrong and seeing that Kel was spending more and more time away from his duties and the people of Dalaran, Antonidas, the leader of the Kirin Tor, investigated. What he found was a chest that contained several rats that were being experimented on, failed experiments. Kel was trying to convert the rats into the Undead, such a cruel thing, the creatures obviously were not willing to have the procedure done and were suffering, trying feebly to escape. Antonidas also found several journals that contained the detailed accounts of the experiments. The taboo works shocked many and were eventually destroyed and Kel'Thalas, seething with anger and frustration, left the Kirin'Tor for good soon after. He sold off many valuable items that he had stored and bought over the years and stashed his amassed wealth for later on. He then travelled across the seas and went to Nothrend to offer his aid to the voice in his head whom had helped him on many occasions, even manipulated him. Travelling over many extreme enviroments of both land and sea by using his wealth he finally came upon Northrend.
Reaching the wretched wasteland a great beast, half-spider and half-Hercules beetle, by the name of Anub'arak, a great Crypt Fiend, led him through the various cold tunnels filled with spider webs. This ancient kingdom led to a great Nercopolis by the name of Naxxramas. There he saw that there were many supplies stored and green liquid that could kill. It did not dawn on him what it was actually supposed to be used for, the supplies and liquid, until later on.
He then was led to a holding area for prisoners. There he found a human male and female, husband and wife. The woman pleaded to be killed and not to be turned into a Undead but Kel did not wish to hear her cries and Anub'arak swiftly killed her.
The woman rose, reborn Undead, and procceded to kill and eat her loving husband after trying to hold back but couldn't. Kel'Thuzad, after witnessing the horror, teleported out and becoming sick, threw up on the snow. He then went to a clean patch of snow and rubbed it all over his body, feeling dirty and guilty for the deaths but not even this would stop him. No, instead he ascended to the Chamber of the Frozen Throne with much persuasion from a few wraiths.
His body, tired from his long journey, was giving out and Kel could barely stand but his bloody and broken body managed to reach Ner'zul on his strong will alone.
Ner'zul, sensing that Kel'Thuzad was wanting to dominate or kill him instead of serve, threatened Kel and made him swear to serve him. Tired, scared, but still hungry for power, Kel accepted his offer. Pleased, the Lich King gave him the power that he wanted. Kel found it to be better than dying, something which he greatly feared, or becoming a mindless servant, something which he also did not desire.
Putting away the Archmage self he once was,he became a Nercomancer, granted with the power from Ner'Zul to raise the dead in perfect form. The Undead he raised were not like the rats that were confused and whos bodies decomposed and muscles refused to function, these were perfect, sublime.
He then travelled to Lordearon on Naxxramas and assembled many of the overworked townsfolks and formed the Cult of the Damned. He then used grain tainted by the fluid coming from vile mushrooms to turn the townsfolk into Undead zombies.
Arthas, the foolish Prince of Terenas, King ofLorderon, killed Kel. Arthas then travelled to Northrend thinking that he could end the Undead once and for all but he was so determined that he could stopt them that he made the mistake of seeking power that would make him turn into the very thing he hated so much and sought to destroy.
Under the guidance of the spirit of Kel'Thuzad and leadership Tichondrious, a DreadLord, Arthas gathered together the acolytes of the Cult of the Damned to summon a base and then he went to a local cemetary to get Kel'Thuzad's remains. Tichondrious then instructs the Death Knight to steal King Terenas' Urn since the meat wagons could not hold the remians intact for long. The reason being that the remains are far too decomposed to make the long journey. Arthas retrieves the sacred urn and puts the remains inside which magically keep it from futher decomposing. Arthas then travels to Quel'Thalas and ravages the land after making it through the sacred Elf gates. After defeating the guards he then proceeds to dip the urn into the waters of the great Sunwell. The energy, along with some help on Ner'Zuls part, ressurrect the necromancer as a Lich but at a great cost for the High Elves whose sun well has been corrupted, thus making their own race tainted.
Seething in immortal power, Kel'Thuzad decides to talk to Archimonde after destroying the Blackrock Orc Clan that was guarding the demon gate and Archimonde instructs the two to find the Book of Medivh and summon him into the world of mortals by reading it. After killing the great Archmages in Dalaran Kel'Thuzad begins the summoning while Arthas protects him. Archimonde, being free in Azeroth, starts corrupting the land. Not long after, Kel'Thuzad was charged with forseeing the land of Lordaeron by his superior, and friend, Arthas.
Now residing in his great ship/Necropolis, the Nexxramas, which floats over Strathlome, Kel'Thuzad keeps his tight hold on the plaguelands and vows to kill all of those who would dare threaten him. He now has his sights set on Sylvannas Windrunner and her group of rebel Undead the Forsaken which desperately try to drive him and his strong forces out.